This invention relates generally to the field of semiconductor wafer fabrication, and more specifically to a system and method for controlling a wafer polishing process.
At various stages during the fabrication of integrated circuits on a semiconductor wafer it is necessary to polish the surface of the wafer. The surface of the wafer is polished to remove areas of high topography and surface defects. The polishing process often involves the introduction of a chemical slurry to increase removal rates and to allow selective removal of certain materials. A polishing process using such a slurry is typically referred to as chemical mechanical polishing (CMP). In the CMP process, the wafer to be polished is affixed to a wafer carrier, and pressed face-down on a rotating platen holding a polishing pad. The slurry, which contains abrasive material, is dripped onto the rotating platen during polishing. The process removes material at the surface of the wafer through a combination of mechanical and chemical action.
The CMP process is subject to variation due to drifts in the polish rate of a polisher over time (caused by wearing of the polishing pad and other factors) and due to varying device characteristics. The latter is caused by differing device layouts (e.g., differing surface designs of fabricated or semi-fabricated wafers), which results in different devices having different apparent polish rates. Because of the accuracy required when manufacturing devices such as integrated circuits, it is necessary to determine the exact amount of time required to polish a layer of a wafer to a desired thickness. However, because of the variations described above, it is difficult to obtain exact polishing times.
In accordance with the present invention, a system and method for controlling a wafer polishing process are provided that substantially eliminate or reduce disadvantages or problems associated with previously developed systems and methods. In particular, the present invention contemplates a control system that simultaneously tracks the polishing characteristics of various device groups across a number of polishers and the polish rate of each polisher.
In one embodiment of the present invention, a method is provided for controlling a wafer polishing process. The method includes determining a polishing characteristic for a particular type of device from a set of measurements obtained from a wafer having a device of the particular type, and updating the polishing characteristic in response to polishing and measuring a wafer having a device of the particular type. The method further includes determining the polish rate of a polisher using a set of measurements obtained from a wafer polished on the polisher; and updating the polish rate of the polisher in response to polishing a wafer on the polisher and measuring the wafer. The method also includes determining a desired polishing time on the polisher for a wafer having a device of the particular type using the updated polishing characteristic of the device and the updated polish rate of the polisher.
In another embodiment of the present invention, a system for controlling wafer polishing is provided that includes a number of polishers that are operable to polish wafers of different device types. The system further includes a number of metrology units that are operable to obtain measurements of a wafer. The system also includes a central control unit that is operable to receive the measurements obtained from the metrology units. The central control unit is further operable to use the measurements obtained from the wafers of a particular device type polished on any of the polishers to determine a polishing characteristic of the device type, and to use the measurements obtained from wafers polished on a particular polisher to determine a polish rate for the polisher.
Technical advantages of the present invention include a system and method for controlling a wafer polishing process that track both polishing characteristics of the different device groups being polished and the polish rate (and change in polish rate) of each of the polishers being used. In one embodiment of the present invention, a local controller is used to track the polish rate of each polisher, and a centralized controller is used to track device characteristics. A key advantage of separating out the device-dependent effects from the polish rate is that lookahead polishes for a new device can be run on one polisher, and the remaining wafers can then be run on a different polisher. Formerly, the polisher had to sit idle while the lookahead was cleaned and measured. The rest of the lot was then allowed to run on the same tool. This idle time is eliminated since the lookahead can run on a different polisher than the rest of the lot.
In addition, the present system and method provide a calculation of the polishing time required for a particular device that is substantially independent of the number of lookaheads that are performed for the device. Furthermore, this calculation explicitly accounts for variation in the thickness of incoming wafers in a device group, and also accounts for errors in the calculated polishing time due to wafer cleaning and measurement delays. The present system and method may also be used with either on-line or off-line metrology (measurement). Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions and claims.